Organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic thin film transistors (OTFTs), and organic solar cells (also referred to as organic photovoltaics or OPVs) based on organic molecules have been extensively studied. Such devices are becoming increasingly popular due to their low-cost processing, relatively simple packaging, and compatibility with flexible substrates.
More specifically, OPVs come with the promise of efficient conversion of sunlight into direct usable electrical energy at a much lower cost than the traditional silicon based solar cells.
OPVs may often contain a mixture of two or more organic materials: at least one material that can act as an electron donor (p-type) and at least one material that can act as electron acceptor (n-type).
Many new materials and new processes have recently been developed for use in OPVs. The most investigated polymeric solar cells are made with regioregular poly(3-hexylthiophene) (P3HT) as donor (p-type) material, which may achieve an efficiency surpassing 5%. However, the main disadvantage of this polymer is the poor matching of its photon absorbance with the solar spectrum. The bandgap of P3HT is around 1.9 eV, limiting the absorption of sunlight to below a wavelength of 650 nm. It has been calculated that with an absorption limit of up to 650 nm only 22.4% of the total amount of photons from sunlight can be harvested.
Since low bandgap polymers can increase the total amount of photons harvested from the solar spectrum, they are potentially more efficient for polymeric solar cells. A low bandgap can be achieved by using alternating donor-acceptor based structures in the polymer backbone. A number of low bandgap polymers have been reported for OPV application.
However, narrowing the polymeric bandgap may result in a decrease of the open circuit voltage, which may eventually result in a decrease in power conversion efficiency. When low bandgap donor polymers are paired with the most commonly used acceptor material, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), the calculated optimal bandgap of the donor polymer is from 1.3 to 1.9 eV for high power conversion efficiency.
The decrease in PCE in OPVs may be due, at least partly, to the fact that, as the absorption band shifts to longer wavelengths, absorption in the short wavelength range may often become weak. Thus shifting to longer wavelengths may not significantly contribute to the overall exciton generation in devices.
Organic semiconductors are also an important component used as channel material in organic thin film transistors (OTFTs). With compared to silicon-based TFTs, OTFTs, especially polymer-based OTFTs, can be fabricated much more cost effectively using solution deposition techniques such as ink jet printing, screen printing, gravure printing, casting, and spin-coating. OTFTs may also offer other attractive features such as flexibility, robustness, and light weight. OTFTs can be used as components in a range of potential applications, which include displays, e-paper, radio-frequency identification tags (RFIDs), bio- and chemosensors. However, the currently available polymer semiconductors suffer low mobility below 0.1 cm2/V·s, which limits the application of OTFTs.
Accordingly, there is a need for production of alternative p-type organic materials useful for production of efficient electronic devices, including OPVs and OTFTs.